New frontiers for the in-situ and operando spectroscopic investigation of interfaces applied to catalysis and electrochemistry

Resume : Catalysts Live & Up Close Bert M. Weckhuysen, Inorganic Chemistry and Catalysis group, Debye Institute for Nanomaterials Science, Utrecht University, Universiteitsweg 99, 3584 CG Utrecht, The Netherlands; b.m.weckhuysen@uu.nl Abstract The search for new or more effective heterogeneous catalysts would benefit when we could bridge the molecular world with the macroscopic world. Such detailed information can be realized if we would have access to a very powerful camera shooting molecular movies of an active catalytic solid at the level of single atoms and molecules. This is the field of operando spectroscopy. Recent breakthroughs in chemical imaging techniques, based on optical, electron and X-ray methods, demonstrate that such molecular movie concept is within reach. This lecture discusses the recent advances in spectroscopy and microscopy of heterogeneous solids at different length scales, starting from single molecules and single atoms up to the level of individual catalyst particles. Special emphasis will be devoted to the exploration of mesoscale effects as well as on the scientific challenges ahead to make such molecular movie reality. It will be shown how this better understanding of catalytic processes may be beneficial for transforming our society to a more sustainable one. The critical concept of such much needed transition towards a circular economy should be: reduce, reuse & recycle.

Resume : Due to the short escape depth of electrons, less than a few nm, photoelectron spectroscopy is the best surface sensitive analytical techniques for probing surface and interface chemical composition, but the investigation of complex systems in electrochemistry and catalysis requires not only near ambient pressure (NAP) conditions, but often the samples are inhomogeneous at the submicron scale. The authors have developed photoemission imaging and spectromicroscopy methodology based on the Scanning PhotoEmission Microscope (SPEM) to simultaneously overcome the two limitations of the XPS technique, i.e. lack of spatial resolution and UHV requirements [1]. This solution allow chemical and morphological analysis providing information under operation conditions. SPEM uses a direct approach to add the spatial resolution to XPS i.e a small focused X-ray photon probe to illuminate the sample. The focusing of the X-ray beam is performed by Zone-Plates and samples surface is mapped by scanning the sample with the focused beam. X-ray beam can be downsized to a diameter of 130 nm, allowing imaging resolution of less than 50 nm with an energy resolution of 200 meV. To overcome the UHV requirement effusive cells where high- and low-pressure regions are separated by small apertures of few hundreds micrometers, for photons delivery and photoelectrons collection, was developed. The pressure inside the cell can be raised up to mbar while the pressure in the main chamber remain in HV. Four isolated electrical connections are available for heating and biasing of sample. Results on the in-situ and operando studies performed at the Escamicroscopy beamline@ ELETTRA synchrotron will be presented [1]. [1] http://dx.doi.org/10.1007/s11244-018-1064-5

Resume : X-ray spectromiscroscopy methods are frequently used for the spatially-resolved in situ characterization of heterogeneous catalysts. However, the choice of the region to be investigated during the measurements and the identification of spectral features during data analysis is often not trivial. Here we demonstrate the use of an unsupervised machine learning technique called cluster analysis to identify and classify regions of interest on the sample surface. Different clustering methods such as k-means clustering, spectral clustering and density-based clustering were evaluated with respect to their success of identifying areas of chemical contrast from scanning photoelectron microscopy (SPEM) and X-ray photoemission electron microscopy (XPEEM) images. It was shown that the use of spectral clustering provides a robust tool for identifying the spatial distribution of complex features with a low signal-to-noise ratio, which are often present in spectral images acquired under near-ambient pressure conditions. This work enables the analysis of spectromiscroscopy data on the fly during fast measurements at the synchrotron, as well as the advanced analysis of convoluted spectral images afterwards.

Resume : X-ray techniques play an important role for gaining the fundamental understanding needed to tailor novel catalysts for CO2 reduction reaction (CO2RR), by providing chemical and structural information of catalytic surfaces. We have utilized surface-sensitive soft and hard X-ray techniques to investigate the interaction of metal catalytic surfaces with electrolytes and/or gases (H2O and/or CO2) under in situ/operando conditions at the Stanford Synchrotron Radiation Lightsources (SSRL) and Advanced Light Source (ALS). We present here our work on Ambient Pressure X-ray Photoelectron Spectroscopy (AP-XPS) for studying CO2 adsorption on Cu and Ag surfaces to understand the initial atomic level events for CO2 electroreduction on the metal catalysts, and CO2 adsorption on Ag/Cu alloys to provide the basis for developing improved catalysts electrolyte/solid catalytic surfaces, as well as surface studies of the catalyst/electrolyte surfaces.

Resume : Recent progress in the field of electrocatalytic CO2 reduction have brought up several classes of novel catalytic systems such as polymers and various metal chalcogenides1,2. These materials possess excellent catalytic activity, but still little is known about the exact catalytic centers and the exact mechanisms leading to efficient turnover. Here we present internal reflection mode infrared spectroscopy as valuable in-situ technique for emerging CO2 electrocatalysts. We demonstrate the exact spectroscopic characterization of the intermediate surface states and conclude on their impact on the Faradaic and energy efficiency. In combination with theoretical modelling in-situ spectroelectrochemistry represents one complementary experimental technique to determine the reaction mechanisms. These insights are useful to develop alternative catalytic materials with improved surface activity and selectivity beyond CO and formate. (1) Zheng, X.; De Luna, P.; García de Arquer, F. P.; Zhang, B.; Becknell, N.; Ross, M. B.; Li, Y.; Banis, M. N.; Li, Y.; Liu, M.; Voznyy, O.; Dinh, C. T.; Zhuang, T.; Stadler, P.; Cui, Y.; Du, X.; Yang, P.; Sargent, E. H. Sulfur-Modulated Tin Sites Enable Highly Selective Electrochemical Reduction of CO2 to Formate. Joule 2017 DOI: 10.1016/j.joule.2017.09.014. (2) Coskun, H.; Aljabour, A.; De Luna, P.; Farka, D.; Greunz, T.; Stifter, D.; Kus, M.; Zheng, X.; Liu, M.; Hassel, A. W.; Schöfberger, W.; Sargent, E. H.; Sariciftci, N. S.; Stadler, P. Biofunctionalized Conductive Polymers Enable Efficient CO2 Electroreduction. Sci. Adv. 2017, 3 (8), e1700686 DOI: 10.1126/sciadv.1700686.

Resume : Direct probing solid-liquid interface is the first step to truly understand an electrochemical system, where the electrochemical interface plays a crucial role. However, it has been a challenge to look through the dense liquid or solid layer and to monitor the thin electrochemical interface while the electrochemical reactions are happening. Many groups have tried to use different in-situ techniques to address this challenge. Particularly, several significant advances have been made recently to probe the electrochemical interface under in situ and operando conditions using synchrotron radiation based characterization tools. Ambient pressure X-ray photoelectron spectroscopy (APXPS) is one of them [1]. In this talk, I will give a brief history how our group developed APXPS techniques using tender X-ray to probe the electrochemical interface directly [2] and show several in-situ studies [3,4] on electrolyte/electrode interfaces and aqueous solutions. At the end, I will share the progress that we have made on X-ray free electron laser development in Shanghai. References [1] X. Liu et. al., Advanced Materials 26 (46), 7710-7729 (2014) [2] S. Axnanda et. al, Scientific Reports, 5,9788 (2015) [3] M. F. Lichterman et. al, Energy & Environmental Science 8 (8), 2409-2416 (2015) [4] M. Favaro et. al, Nature communications 7, 12695 (2016)

Resume : Ambient pressure hard x-ray photoelectron spectroscopy (AP-HAXPES) has been used to study the interaction of several electrolytes with the surface of bismuth vanadate (BiVO4), a technologically relevant water-splitting photoelectrode material. Structural changes in the surface and electrolyte were seen under illumination, which were dependent on the chosen electrolyte. The development of ambient pressure X-ray photoelectron spectroscopy (AP-XPS) has enabled the study of material surfaces at realistic pressures. While many studies have used soft x-ray XPS to study solid-gas interfaces, the attenuation length of electrons at this energy range can be limiting. The recent application of the tender or hard x-ray energy range to ambient pressure XPS (i.e. AP-HAXPES) allows the investigation of buried interfaces, including solid-liquid interfaces. We have applied this technique to the BiVO4 – electrolyte interface. In this study, a liquid layer of electrolyte was formed on the surface of thin film BiVO4 by the dip-and-pull method, and the resulting solid – liquid interface probed by AP-HAXPES under illumination by a solar simulator. Using potassium phosphate, we observed spectral changes consistent with the formation of bismuth phosphate, and concurrent restructuring of the electrolyte. Like changes were absent when illuminating a similar layer of potassium borate. This study offers insight into the effect of electrolyte choice on device stability and demonstrates the value of AP-HAXPES as a tool for probing solid-liquid interfaces.

Resume : Cu(In,Ga)Se2 (CIGS)-based photovoltaic cells have been demonstrated to be excellent bottomcells in perovskite-based tandem devices for enhanced power-conversion efficiency [1,2]. However, the low temperature resilience of the standard CIGS/CdS interface limits the choice of materials and further processing window for the top cell. This study aims to analyze the degradation mechanisms responsible for the premature deterioration of the buffer and interface. Additionally, an alternative Zn(O,S) buffer-layer and possible effects of a RbF post-deposition treatment (PDT) on either of these interfaces are investigated. The study employs in-situ, synchrotron-based hard X-ray photoelectron spectroscopy. The substrate temperature was slowly ramped up (0.5°C/min), covering a range from 150°C-350°C, which results in a quasi-continuous temperature resolution. The measurements covered all relevant elements (Cu, In, Ga, Se, Na, Cd, Zn, O, S), ensuring that degradation temperature and mechanism can be identified. RbF PDT delayed the buffer deterioration for CdS buffers, while the degradation of Zn(O,S) buffers was found to be accelerated. Furthermore, RbF PDT-induced Na depletion of the absorber and delayed Na diffusion are found in this study. [1] Qifeng Han Science 31, 2018:, pp. 904-908 DOI: 10.1126/science.aat5055. [2] M. Jošt et al., ACS Energy Lett. 4, 2019, pp 583–590 DOI: 10.1021/acsenergylett.9b00135.

Resume : To probe active electrocatalyst surfaces in a liquid environment, we have developed an XPS/XAS cell in which the catalyst is confined between a proton exchange membrane and graphene. While the proton exchange membrane supplies a steady flow of electrolyte to the electrode, the X-ray and electron-transparent graphene layer greatly reduces the evaporation of water into the NAP-XPS chamber. We show that this can lead to the formation of a thin layer of liquid electrolyte between the graphene and the membrane. Thus, electrocatalysts can be studied under operating conditions using surface sensitive soft X-ray XPS and XAS [1]. With this methodology, we have studied the potential-driven restructuring of Ru, Ir and Au oxides in 0.1 M H2SO4 during the oxygen evolution reaction. While for AuOx only Au3+ is found, the electronic structure of the Ru and Ir cations is dynamic, particularly for Ru. O K-edge spectra, complemented by theory, indicate that the oxidation of the catalysts occurs through deprotonation, even in the bulk of the materials. The deprotonation proceeds through multiple stages: hydroxyl groups with higher coordination deprotonate at lower potential. Interestingly, we find that deprotonation is not complete during the oxygen evolution reaction on Ru and Ir. [1] Mom et al., J. Am. Chem. Soc., 2019, 141 (16), pp 6537–6544

Resume : The investigation of processes at the solid/liquid or solid/gas interfaces is of paramount importance in many fields, including catalysis, electro-chemistry and energy. Synchrotron-based spectroscopic techniques, such as X-Ray Photoelectron Spectroscopy (XPS) and X-Ray Absorption Spectroscopy (XAS) can offer a comprehensive understanding of the electronic properties of materials in atmospheric conditions or in liquid environment, providing information on the electron transfer processes at the interfaces during catalytic reactions. Cells based on graphene and Si3N4 membranes have been demonstrated to work as soft X-ray transparent interface between liquid and vacuum. Sealed graphene nanobubbles (GNBs) on a titanium dioxide TiO2 (100) rutile single crystal filled with different aqueous solutions were developed and characterized by AFM, Raman and synchrotron radiation spectroscopies. GNBs were successfully employed to follow in-situ two reactions: thermal-induced reduction of FeCl3 to FeCl2, and photo-induced reduction of ferromagnetic Prussian Blue to paramagnetic Prussian White in aqueous solution. The electronic states of iron atoms in both systems and the presence of solvent were obtained through a combination of XPS and XAS measurements, which allowed the comprehension of the electron reduction mechanisms and the role of water in both processes. A cell, equipped with a 100 nm thick Si3N4 window, was developed to perform in-situ XAS measurements in liquid phase. The effect of the solvent in the unoccupied electronic structure of iron and cobalt atoms, as well as the changes in the local charge and coordination state of CoFe2O4 nanoparticles suspended in aqueous solution were investigated by fluorescence yield XAS.

Resume : Here we demonstrate the proof of principal and initial results of performing operando XPS electrochemistry in a lab based near atmospheric pressure (NAP) XPS system utilising a standard lab based monochromatic aluminium source. A thin capillary is used inside the NAP-Cell in conjunction with a simple custom built electrochemistry cluster in order to advance and recede droplets onto a sample surface. Small volumes of solution have been dispensed on surfaces in vacuum and ultra-thin liquid layers (<10 nm) have been shown to allow analysis of the solid/liquid interface all whilst allowing electrochemical control, thereby creating a lab based operando XPS electrochemical set-up. Novel features such as in-situ sample rinsing and the ability to modify the chemical properties of the electrolyte during analysis by pulsing various liquid solutions into the analysis droplet boost the appeal of this technique when compared to other available operando techniques. Analysis of the solid/liquid interface has been performed on a number of samples such as TiO2, Pt and Fe in addition to analysis of bulk electrolyte solutions.

Resume : Over the last decades XPS under Near Ambient Pressure (NAP) conditions has demonstrated its promising potential in a wide variety of applications. Starting from operando studies of surface reactions in catalysis, the applications soon have been enhanced towards studies of processes at liquid surfaces, mainly using freezing/melting cycles, liquid jets or liquid films on rotation disks or wheels. Since more than 15 years, the need for basic studies of the fundamental chemistry at solid-liquid interfaces has attracted growing interest. Dip-and-pull experiments at synchrotrons finally also demonstrated, that in-situ and operando XPS in electrochemical experiments can be realized, mainly using synchrotron beams, significantly contributing to the basic understanding of modern energy converting or storing devices, like batteries, fuel cells, etc. The development of pure laboratory NAP-XPS systems with optimized sample environments, like special sample holders, Peltier coolers and operando liquid cells combined with full automation and process control provides possibilities for the preparation and analysis of a multitude of liquid samples or solid-liquid interfaces on a reliable daily base. Interfaces of semiconductors with organic solvents are important for production processes and device operation. The first example presented shows the simplicity of obtaining relevant results on Silicon in different organic solvents without the need of highly sophisticated setups or special excitation sources beyond Al K alpha. The next level of complexity is to follow the effects of corrosion in organic or inorganic acids. As an example an operando study of metal corrosion in acetic acid is shown. Most sophisticated experiments so far have been operando electrochemistry in a classical three-electrode setup. A versatile setup is presented, allowing for studies of solid-electrolyte interfaces for example in Lithium ion batteries as a simple laboratory experiment. Finally an outlook is given on the future perspective of applications and scientific contributions of routine operando XPS.

Resume : Converting solar or electric energy into chemical energy contained in the hydrogen bonds is a promising approach for long time storage of energy. However, the Oxygen Evolution Reaction (OER) limits the conversion efficiency of this process, because even the overpotentials of the best-known catalysts for water splitting remain quite high. To overcome this obstacle, better suited materials containing inexpensive, corrosion-resistant, and abundant elements are required. For rational design of new OER catalysts, an in-depth comprehension of the underlying conversion processes at the solid liquid interface is crucial. We apply operando photoelectron spectroscopy (PES) that reveals details of the OER mechanism of well-known catalytically active transition-metal oxides, e.g. iron-nickel oxy-hydroxides. We will present first experimental results from our newly developed (photo)electrochemical flow-cell. It is equipped with a bi-layer graphene membrane, acting as the working electrode, on top of a few-nanometer transition-metal oxide catalyst film – both transparent enough to electrons above 300 eV kinetic energy and to soft X-rays – that allows us to study the solid-liquid interface at a defined electrode potential using the Bessy II synchrotron soft X-ray source. Our method of choice is resonant PES, which displays details about the metal-water hybridization at the interface, by detecting the resonant Auger electron emission at the transition metal L-edge and water oxygen K-edge.

Resume : Electrochemical attenuated total reflectance surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) was first introduced by Osawa [1] and co-workers and has emerged as a powerful tool for probing electrified interfaces. Silicon is often the material of choice for the internal reflection element (IRE) due to its wide pH stability and its compatibility with various metal thin film deposition techniques. Because silicon suffers from inherent absorptions in the fingerprint region, the path length through the IRE becomes a critical parameter in achieving high throughput. A recent alternative to usual cm-sized IREs are the microstructured wafers reported by Schumacher et al. [2] and further developed by the Burgess group [3]. Grooves, formed by wet chemical etching of a 500 μm thick Si wafer, allow the use of the wafer as a single-reflection IRE. Due to the considerably shorter path length, the accessible wavelength window is wider compared to conventional Si IREs. Here we present an ATR-SEIRAS setup for investigations of the electrochemical double layer at Au thin film electrodes based on low-cost Si wafers. Two different microstructures with different groove angles are compared in terms of spectral intensity of potential-dependent operando SEIRA spectra, collected during electrochemical measurements in sulfate and bicarbonate electrolyte. [1] Bull. Chem. Soc. Jpn.1997, 70 (12), 2861-2880. [2] Appl. Spectrosc. 2010, 64, 1022−1027. [3] Anal. Chem. 2017, 89, 11818-11824.

Resume : Gallium nitride (GaN) semiconductor has numerous featured physical and chemical properties that make it a promising candidate for use in biomolecular-based microelectronic devices and biosensors because of its natural living-cell biocompatibility and potentiality to convert biological information directly into electrical signal;however, evaluation and assessment of GaN surfaces performance in such devices can be achieved via studying the chemical structure. In this study, the chemical structure and morphology of undoped GaN(0001) surfaces on sapphire substrates were investigated for different treatment procedures and in dry/wet-water conditions using a variety of surface sensitive techniques.

Resume : Until recently, most ambient pressure x-ray photoelectron spectroscopy (APXPS) experiments have focused on the study of solid-gas interfaces. The recent extension of APXPS measurements to above 30 mbar opens the possibility to study aqueous solutions at their room temperature equilibrium vapour pressures. In combination with tender X-ray (2-12 keV) photoemission spectroscopy, solid-liquid interfaces beneath liquid layers of several 10s of nms can be probed1. This technique can be used to study electrochemical interfaces ¬in situ and -operando, under light conditions and applied potentials. Additionally, the study of the surface region of liquids can be achieved with a droplet train – a continually refreshed, stable source of uniform droplets2. Because of their stability, droplet trains offer the possibility to study the kinetics of chemical reactions, and the nucleation and growth of nanoparticles in solution. We present the capabilities of a new APXPS endstation based at the BESSY II synchrotron, designed for studying solid-liquid, liquid-vapour, and liquid-liquid interfaces. It has two interchangeable modules: the first for solid-liquid interface experiments (commissioned) and the second for droplet train experiments (in development). SpAnTeX is equipped with the first Phoibos 150 NAP analyzer that can reach photoelectron kinetic energies of 10 keV. We show the attenuation of the photoemission signal upto pressures of 30 mbar for photoelectron kinetic energies up to 8 keV, and measurements at 10 keV in 25 mbar of N2 gas. The analyzer is also equipped with a 2D-delay line detector, allowing time-resolved photoemission measurements in ambient conditions. This permits observations of chemical kinetics over time ranges from nanoseconds to seconds and longer. The analyser can also be used in an imaging lens mode, with a lateral-spatial resolution in the 10s of microns, under ambient conditions. Possible applications for this include the study of patterned electrodes and determining spatial differences in droplet reactions. 1 M. Favaro, F. F. Abdi, E. J. Crumlin, Z. Liu, R. Van De Krol and D. E. Starr, Surfaces, 2019, 2, 78–99. 2 D. E. Starr, E. K. Wong, D. R. Worsnop, K. R. Wilson and H. Bluhm, Phys. Chem. Chem. Phys., 2008, 10, 3093–3098.

Resume : H2-based energy has long been regarded as a promising alternative to conventional fuels. One of the efficient methods for H2 production (CO free) is low temperature methanol steam reforming (MSR). The catalysts most commonly used for this process are copper based catalysts due to their high activity and low CO production. In connection with the active species in Cu-based catalysts for MSR, there is still ambiguity about their nature1. Few studies have evaluated structural changes of the catalyst under working reactions, specifically those taking place in the initial stage of the reaction. In our study2, the dynamic behavior of a Cu65Zn25Ga10 catalyst under MSR reaction conditions was found by the application of temporal resolution catalytic studies and synchrotron near ambient pressure x-ray photoelectron spectroscopy (NAP-XPS). Surface copper atoms remain in a metallic state while they are electronically perturbed by the presence of subsurface oxygen species. These species influence the catalytic behavior of the system in MSR, increasing its performance. The presence of these subsurface oxygen species is due to the fast oxidation and further reduction of the copper particle during the first minutes of the reaction, and results undetected using a conventional setup. Thus, the use of spatiotemporal resolved spectroscopies with in-situ capabilities (during reaction conditions) are needed to fully understand the catalytic behavior of a catalyst. 1 GS. Wu, DS. Mao, GZ. Lu, Y. Chao, KN. Fan. Catal. Lett. 2009, 130, 177-184. 2 D. Ruano, J. Cored, C. Azenha, V. Pérez-Dieste, A. Mendes, C. Mateos-Pedrero, P. Concepcion. ACS Catal. 2019, 9, 2922-2930.

Resume : The CNR Beamline for Advances Circular diCHroism (BACH) operating at Elettra in Trieste (Italy), works in the VUV-soft x-ray photon energy range with selectable light polarization, high energy resolution, brilliance and time resolution . The beamline offers a multi-technique approach for the investigation of the electronic, chemical, structural, magnetic and dynamical properties of materials. One of the three end-stations has been recently dedicated to experiments based on electron transfer processes at the solid/liquid interfaces during photocatalytic or electrochemical reactions. This goal has been pursued by the development and implementation of new strategies. The encapsulation of liquid solutions between a graphene layer and a solid substrate was successfully applied to the study of thermo and photo-induced reactions by X-ray photoemission spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). A second approach is the use of a microfluidic cell, consisting of a Si3N4 membrane which separates the liquid from the vacuum and equipped with a three electrodes systems, where the working electrode (WE) is the membrane itself. The electrochemical cell allows to carry out cyclic voltammetry in situ or electrochemical deposition on the WE and chemical reactions in-operando conditions. XAS in fluorescence is used for the operando characterization, providing unique information with elemental sensitivity of the chemically active atomic orbitals of the material. We plan to implement the use of hole array membranes covered by graphene layers in order to have access also to XPS. The availability of a fs laser synchronized with the x-ray beam will allow pump and probe XAS experiment in liquid environment in-operando conditions.

Resume : Electrochemical reduction of carbon dioxide can produce a wide variety of products with poor control over selectivity and poor efficiency. The nature of the catalyst surface plays a central role in dictating the reaction activity, and researchers are working to develop new catalysts and uncover design principles enabling selective and efficient production of a desired product. A key challenge is that electrocatalyst structure is highly dependent on the applied electrochemical potential and the chemical environment, especially under the particularly reducing conditions required for CO2 reduction. Methods for in situ study of electrodes under operating conditions are therefore crucial. In seeking to modify catalyst activity by surface functionalization, we discovered that sub-nm ALD coatings of SnO2 films onto CuO nanostructure electrodes led to huge changes in catalyst selectivity, converting CO2 to CO (carbon monoxide) with selectivity exceeding 90%. Since this behavior is atypical for either CuO or SnO2 electrodes themselves, a synergetic effect between the two components is likely. We employed X-ray spectroscopy to investigate the electronic states and chemical environments of Cu and Sn using soft and hard X-ray techniques at BESSY II. We discovered that, during electrochemical operation, the Sn migrates away from the surface and into the CuO bulk. The presence of Sn influences the observed oxidation state of Cu near the surface, which remains partially oxidized (in comparison to fully reduced Cu which is observed in the absence of Sn). This result shows that electrocatalytic surfaces can be significantly affected by the presence of trace sub-surface dopants, causing changes in electronic structure which affect catalytic mechanisms and the resulting product selectivity.

Resume : The chemical industry is one of the most important sectors of our economy and it is estimated that nearly 90% of all chemical products rely on suitable catalysts that enhance the rate of chemical reactions. In order to generate a sustainable society, hydrogenation processes related to CO and CO2 are of great importance to generate fuels. However, the related processes take place under elevated pressures of several bars and increased temperatures above 100°C. Some of the most important information about the catalytic process is related to the bond breaking and bond formation of molecules at interfaces. In order to gain insight into the mechanistic details, we have developed a new high-pressure x-ray photoelectron spectroscopy setup capable of acquiring spectra at pressures exceeding 1 bar and temperatures up to 500°C under operando conditions. The instrument takes advantage of a “virtual cell” which is a novel concept in which a gas stream is pointed towards the sample surface to create a localized high-pressure pillow. Synchrotron based hard x-ray excitation is used to enhance the inelastic mean free path of the electrons inside the high-pressure gas environment. We use grazing incidence spectroscopy for efficient detection of surface species, using a precision motion manipulator, capable of scanning the incidence angle with step sizes of 2urad. Details on the instrument and first measurement results obtained at beamline P22 at Petra III will be presented.

Resume : The combination of complementary techniques in the characterization of catalysts under working conditions is a very powerful tool for an accurate and in-depth comprehension of the system investigated. In particular, X-ray absorption spectroscopy (XAS) coupled with diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and mass spectroscopy (MS) is a powerful combination. In the present contribution, a new reactor cell and an experimental setup are presented. The system stands out among others for its achievements and flexibility. The reactor cell is made up of the main body, where the heater and the reactive gas pipes are placed, sharing different domes can be placed. This solution allows to optimize the setup for different applications and future development with only minor adaptation. A key feature of this setup is the possibility to work at high temperature and pressure, with a small cell dead volume (up to ≤ 0.5 cm 3 ) to perform time-resolved experiments on heterogeneous catalysts under working conditions. XAS measurements can be performed in transmission or fluorescence modes, and the cell is compatible with external devices like UV-light and Raman probes. To demonstrate these capabilities, performance tests with and without X-rays are performed. The effective temperature at the sample surface, the speed to purge the gas volume inside the cell and catalytic activity have been evaluated to demonstrate the reliability and usefulness of the cell.

Resume : Interfaces are the place where most of the chemical reactions occur, but are intrinsically difficult to investigate, because of their nano-confinement. Thanks to its surface sensitivity and to the possibility to investigate real samples in the millibar pressure range, x-ray photoelectron spectroscopy can be considered a promising technique. This poster will describe a new beamline at the Swiss light source synchrotron (Paul Scherrer Institute), where ambient pressure x-ray photoelectron spectroscopy and near edge x-ray absorption fine structure spectroscopy can be carried out to characterize in situ reactions at the solid-gas and at the solid-liquid interface. The solid-gas interface endstation allows the characterization of actual samples (powders) exposed to reactive gas environments. Thanks to the small volume of the analysis cell, pulsed experiments are possible, so that the electronic properties of samples can be tested under transient reaction conditions with a time resolution in the second range. Depending on the research topic, it is possible to control the sample temperature either in the high temperature range (room temperature to 650°C) or in the low temperature one (100°C to -100°C). This makes the setup particularly suitable for catalysis and environmental chemistry experiments. The solid-liquid interface chamber has just been commissioned. In this setup it is possible to stabilize a thin liquid film on top of a solid sample by a dip&pull method. Using tender X-rays, we can probe the solid-liquid interface collecting photoelectrons emitted from the buried interface. A customized three electrode system allows the precise control of the potential over the electrolyte film, making the setup suitable for electrochemistry experiments.